The present invention relates to an exhaust emission control device for an internal combustion engine.
Some catalysts for purifying the exhaust gas of an internal combustion engine absorb NOx in exhaust gas when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. When the oxygen concentration of the exhaust gas falls, i.e. when the air-fuel ratio changes over to stoichiometric or rich, the NOx absorbed on the catalyst is desorbed, and the desorbed NOx is reduced by HC and CO which are present in the exhaust gas (JPA 6-336916 published in 1994).
However, engine fuel or lubricating oil generally contains sulfur, and SOx (sulfur oxides) in the exhaust gas tends to be absorbed by or deposit on the catalyst more easily when the vehicle is being driven for long periods of time at a lean air-fuel ratio. If a large amount of SOx is deposited on the catalyst, absorption of NOx declines and exhaust gas purification performance is considerably impaired.
The SOx deposited on the catalyst is discharged from the catalyst when the catalyst temperature rises above its usual level. Hence, when the amount of SOx deposited on the catalyst increases, the catalyst temperature is increased to remove SOx.
In JPA 10-54274 (published in 1998), when the amount of SOx deposited on the catalyst increases and absorption of NOx by the catalyst declines, lean misfire of the engine is performed for a predetermined time. Due to this, the level of partial combustion increases in the exhaust gas, the temperature of the catalyst increases when these unburnt components are burnt in the catalyst, and SOx is discharged.
Alternatively, the ignition timing of the engine is retarded to increase the temperature of the exhaust gas and discharge SOx from the catalyst.
However, although the temperature increase effect is large when a lean misfire is performed and unburnt components are burnt in the catalyst, it is difficult to correctly control the amount of unburnt components sent to the catalyst, and there is a possibility of the temperature rising too much and impairing the durability of the catalyst.
On the other hand, since the temperature increase due to the retardation angle of the ignition timing raises the catalyst temperature indirectly by raising the exhaust gas temperature, durability is maintained but a sufficient temperature increase for discharging SOx is not necessarily achieved.
A method is known of improving exhaust gas composition immediately after engine startup by installing a three-way catalyst on the upstream side, i.e. in the exhaust manifold, where the temperature after startup increases quickly. However, if an NOx absorption catalyst is also installed downstream of the upstream three-way catalyst and it is attempted to raise the temperature in order to discharge SOx from the downstream catalyst, the temperature of the upstream catalyst increases too much, and these severe temperature conditions lead to early deterioration of the upstream catalyst.
It is therefore an object of the present invention to provide an exhaust emission control device for an internal combustion engine which increases the temperature of a downstream catalyst effectively while avoiding the temperature increase of an upstream catalyst.
In order to achieve above object this invention provides an exhaust emission control device of an internal combustion engine provided with an exhaust gas pipe, comprising a front three-way catalyst disposed in the exhaust gas pipe, a rear three-way catalyst disposed in the exhaust gas pipe, the rear three-way catalyst being provided downstream of the front three-way catalyst, and a microprocessor. The microprocessor is programmed to control the engine so that an air-fuel ratio of the exhaust gas flowing into the front three-way catalyst is made to periodically fluctuate to rich or lean about the stoichiometric air-fuel ratio, and increase the amplitude of air-fuel ratio fluctuation when the temperature of the rear three-way catalyst is raised.